In the related art, the output current or the output voltage in a switched-mode power supply is regulated in the current mode or in the voltage mode.
In the current mode, the switched-mode regulator operates according to the following principle: a peak current setpoint that is to be reached in the transformer is selected. Then a certain pulse width is established in the transformer, depending on the setpoint, as illustrated in FIG. 1. This shows the current I in the transformer, the setpoint of the peak current I.sub.s and the resulting pulse width p.
It can easily be seen that the pulse width p can very rapidly assume large values in transient processes, so the transformer runs into saturation after only a few cycles because it is no longer completely demagnetized at large pulse widths. One could of course limit the pulse width p to a fixed value, but this has the disadvantage that long-range power supply units can hardly be implemented.
In the voltage mode, the selected setpoint is not a peak current but instead it is the pulse width p directly (FIG. 2). This is generated by comparison of a delta voltage or a saw-tooth voltage with the setpoint. However, here again there is the same problem that the pulse width p can vary too much in transient processes, and then the transformer no longer demagnetizes. FIG. 2 shows the three signals in the voltage mode: delta voltage U.sub.D, setpoint U.sub.s and pulse width p.
The object of the present invention is to find a possibility of limiting the pulse width according to the single-ended flux converter principle in a switched-mode power supply so that saturation phenomena in the transformer and the associated overvoltages are prevented.